The present invention relates generally to medical imaging systems, and more particularly, to a phased array coil for a magnetic resonance (MR) imaging system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Over time, MR systems have progressed from low-field, single-channel systems toward high-field multi-channel systems, allowing highly accelerated parallel imaging. Despite continuous advances in conceptual understanding and MR hardware design, high-field imaging still poses significant challenges in terms of patient-dependent RF-wave interference effects (i.e., receiving sensitivity inhomogeneities in the form of RF-shading). With increasing B0 and object size, the RF fields change from a quasi-static regime into a more wave-like regime and thereby become increasingly determined by the subject's properties. That is, a subject's size, shape and dielectric properties can affect RF fields, causing RF-wave interference effects. These RF-wave interference effects pose significant challenges for the design of high field coil arrays, which are aimed to consistently achieve uniform, high signal-to-noise ratio (SNR) performance, when applied to a broad patient population under various conditions.
MR receiver-coil arrays often use equally sized coil elements, with the center of the array co-aligned with the subject's inferior-superior axis. These arrays having equally sized coil elements may suffer from shifting of coil sensitivity patterns relative to the subject, although such shifting may be minimal for B0 values of 1.5 T or lower. This shifting often stems from the local intensity shift artifact (LISA). For larger B0 values, such as 3 T or higher, MR images acquired from these receiver coil arrays having equally sized coil elements often show greater variation in SNR in the plane transverse to the B0 field, which causes shading in the MR image and degrades overall image quality. The direction of the shading depends on the direction of the static B0 field relative to the subject and the placement of the receiver coil array. Take, for example, a B0 field pointing from the subject's feet toward the subject's head and a conventional coil array placed underneath the supine subject. In such an example, the left side of an acquired coronal image, which corresponds to the subject's right side, is typically brighter than the right side of the coronal image. In addition, shading effects can also be seen along the anterior-posterior axis of the MR image. If, on the other hand, the B0 direction is reversed, pointing in the subject's head-to-feet direction, the right side of an acquired coronal image is typically brighter than the left side. In addition, as with the first example, shading effects will also be present along the anterior-posterior axis relative to the image. These shading effects may present themselves to some degree for magnetic field strengths of 1.5 T or lower; however, the effect becomes more severe as field strength increases.
It would therefore be desirable to have a system and apparatus that increases SNR uniformity, thereby reducing shading artifacts.